Large, single-piece collection optics in telescopes are difficult to is manufacture and suffer from physical instability, e.g., sag. Telescopes making use of large primary mirrors formed from separate mirror segments have been proposed and implemented. Examples of such telescopes include the Keck Telescope.
The mirror segments in a segmented mirror array are desirably co-aligned so as to ensure better imaging. Such co-alignment is referred to as “phasing” and requires the removal of tip, tilt, and piston errors of each segment relative to a reference segment. Because segmented mirror telescopes include multiple, movable segments that may engage in unwanted movement, these systems require persistent alignment between segments to prevent image degradation.
Piston error is more difficult to measure with conventional interferometry than tip and tilt error. The difficulty arises from the periodic nature of monochromatic interference patterns. Monochromatic interference patterns produce an integer number of waves of ambiguity associated with the piston step between mirror segments. The difficulty is also compounded by the fact that piston step should be corrected to high precision.